1. Field of the Invention
The present invention relates to a method and apparatus for feeding back Channel Quality Information (CQI) in a Multi-User Multi-Input Multi-Output (MU-MIMO) communication system.
2. Description of the Related Art
Recently, in the field of wireless communication systems, many studies have been conducted on Down-Link (DL) channels in a Multi-User Multi-Input Multi-Output (MU-MIMO) communication system in which one base station supports multiple users simultaneously. In the DL channels, inter-user interference is a large problem. To address this problem, Zero-Forcing (ZF) beamforming and Block Diagonalization (BD) for designing a precoder have been widely used. In the precoder design, acquiring correct information about DL channels by a transmitting side is a precondition. Based on this information, a receiving side performs a user's scheduling algorithm, making it possible to obtain a higher sum-rate performance.
In a wireless communication environment, however, it is difficult for a transmitting side to acquire correct information about DL channels. Therefore, a receiving side quantizes information about a channel to a transmitting side based on a codebook it has recognized in advance, and transmits, to the transmitting side, Channel Direction Information (CDI) which is a quantized codebook index. The CDI represents a direction of the channel. Upon receiving the CDI, the transmitting side may design a precoder for cancelling interference of the channel based on the CDI.
In the MU-MIMO communication system, if multiple users exist, performance gain or multi-user diversity gain may be obtained by assigning users appropriate for the transmission environment. To this end, a user transmits to the transmitting side not only the CDI for its own channel but also Channel Quality Information (CQI) corresponding to characteristics of the channel. Upon receiving the CDI and CQI transmitted from the user, the transmitting side may perform more efficient scheduling based on the CDI and the CQI.
In one scheme according to the related art, when a receiving side with one antenna performs scheduling in a vector channel, this scheme analyzes ZF beamforming performance. In another scheme according to the related art, when a receiving side has multiple antennas, this scheme obtains performance gain by reducing errors caused by channel quantization by means of combining at the receiving side. In a further scheme according to the related art, a scheduling gain is obtained by reflecting quantization errors in CQI. The above schemes according to the related art have constraints in which each user should perform communication only through one data layer, i.e., should perform single-mode communication.
In a proposed scheme, during multi-mode communication in which a transmitting side supports multiple data layers for a user, this scheme uses the BD technique. In this case, the BD technique is used based on only the CDI, for a specified user, and the CQI representation technique for multi-user scheduling has never been considered. A user may measure its channel magnitude, and use the measured channel magnitude as CQI. In this case, in a limited feedback situation, quantization errors may not be reflected, causing performance degradation.
Therefore, in the MU-MIMO communication system, when channel information is transmitted to a transmitting side using a codebook, CQI representation distinguishable from that of a system supporting only one user is required.